The present invention relates to a device and method for removing relatively uncontaminated CRUD samples from nuclear fuel assemblies, and more particularly, from nuclear fuel rods, for analysis.
Pressurized water reactors (xe2x80x9cPWRsxe2x80x9d) which are used to generate nuclear power include reactor cores that have fuel assemblies. Fuel assemblies are usually are made up of elongated hollow metallic tubular cladding, known as xe2x80x9cfuel rods,xe2x80x9d that contain nuclear fuel. Cladding functions to prevent fission products from being released from the fuel into the coolant/moderator and to prevent contact and chemical reactions between the fuel and the coolant/moderator. Common cladding maters include zirconium, zirconium alloys, and stainless steel. Zirconium based alloysxe2x80x94in which the major component is zirconiumxe2x80x94have been used in the cladding of nuclear fuel rods or elements for several decades.
Irradiated fuel rods that have been expended in a reactor core are often stored in water-filled pools for deactivation. As time passes, CRUD collects over an oxide layer which forms on the nuclear fuel rods. CRUD can also collect on rods while they are in use. CRUD is an acronym for xe2x80x9cchalk river unidentified deposit,xe2x80x9d referring to highly radioactive substances originally found on the inside of piping and components at the Chalk River Nuclear Reactor in Ontario, Canada. CRUD has now become standard nomenclature referring to minute, solid, corrosion products that travel into the reactor core, become highly radioactive, and then flow out of the reactor into other systems in the plant. CRUD can settle into crevices or form coatings on piping, fuel rods, and other surfaces in a reactor. The major components of CRUD are iron, cobalt, chrome, and manganese. CRUD is known to be a concentrated source of radiation and represents a significant radiological risk because of its insolubility. Therefore, not surprisingly, nuclear plant workers are known to receive radiation doses directly from CRUD exposure during refueling and maintenance outages. As such, it is important to analyze CRUD samples to determine the nature and amount of the deposits. Specifically, the samples can be analyzed to determine their elemental and isotopic composition and to provide an estimate of the activity of the CRUD material resident on the fuel rods.
It is known in the art to use an essentially manual device to retrieve a CRUD sample from nuclear fuel rods. Such a device usually consists of a scraping head at the end of a manually operated pole. The scraping head is often a stone substance that crumbles during use and can score the surface of the cladding. Additionally, biological protection is not typically provided in such a system itself. This can lead to handling difficulties both technically and in terms of governmental regulations.
Also known are mechanized devices for removing deposits from irradiated fuel elements. For example, one such device is disclosed in U.S. Pat. No. 4,483,205 to Bellaiche et al. This patent describes an apparatus that scrapes fuel rod surfaces by means of a remote-controlled scraping device. The scraped sample is sucked into an intermediate reservoir along with liquid from the pool. The samplexe2x80x94carried by the liquidxe2x80x94is passed into a filtration chamber that retains the sample. The liquid can then be passed back into the pool.
Prior art devices for collection of CRUD suffer from a number of shortcomings including, for instance, sample contamination, relatively low sample volume, and cladding scoring. Contamination occurs because pool water is used as the transportation media for the sample. The chemical composition of pool water can vary, and the water can also contain high levels of certain elements that can result in reduced accuracy and resolution of the sample. Contamination has also been shown to occur as a result of the crumbling of the consumable scraping stone during its use. The stonexe2x80x94which itself may include components of interest during analysisxe2x80x94can cause further contamination if it mixes with the deposit sample. Moreover, scraping stones often fail to cover a relatively sizeable area of the rod and become clogged. Each of these attributes limit the scraping efficiency of the stone and necessarily reduces the amount of CRUD removed from the rod. Stones are also known to score the surface of the cladding.
Thus, there remains a need for a CRUD collection system that overcomes the limitations of prior art devices.
It is therefore an object of this invention to provide a collection system that allows for the collection of a relatively uncontaminated sample of CRUD from an irradiated fuel rod.
It is another object of this invention to provide a collection system that allows for the collection of a relatively large sample of CRUD.
It is yet another object of this invention to provide a collection system that scrapes fuel rods to obtain CRUD samples without doing damage to the fuel rods.
To this end, the present invention provides for a CRUD collection system for removing deposits from a nuclear reactor fuel rod submerged in pool water. The system includes a sealable collection head for containing a portion of the fuel rod. The collection head includes at least one seal positioned to receive the fuel rod. Also provided is a liquid inlet communicating with an interior portion of the collection head, the inlet is configured to inject a liquid into the interior to displace the pool water therefrom. At least one blade is provided that is positionable in scraping engagement with the fuel rod. Also included is a liquid outlet for eliminating a CRUD sample and the liquid to a filter.
The system as described above, has been found to provide an exemplary means for collecting CRUD wherein the CRUD sample is uncontaminated by pool water. The system further provides a sealable chamber for enclosing part of a rod so that the rod can be surrounded by deionized water and subsequently sampled in a substantially contaminant-free environment.
Also provided is a method for collecting a CRUD sample that begins by removing a fuel rod from a fuel assembly. Next the fuel rod is inserted into a collection head. The collection head is closed and is then flushed with deionized water to remove pool water. The rod is then scraped with at least one scraping blade to remove the CRUD sample. The deionized water is filtered to remove the CRUD sample. The filter membrane is then removed so that analysis can be performed on the trapped CRUD sample.
Other objects, features and advantages of the present invention will become more apparent from the following description of the preferred embodiment of the present invention when considered in connection with the accompanying drawings.